Field of the Invention (Technical Field):
A process for the preparation of multifunctional polycarbodiimides, which are used as crosslinking agents, in which the polycarbodiimide is prepared by reacting mono- and polyisocyanates in the presence of a mono- or polyisocyanate that contains one or multiple additional reactive functional groups and in the presence of a carbodiimide catalyst, and thereafter terminating or chain extending the polycarbodiimide chain. Further, the invention relates to a coating mixture in which the polycarbodiimide is used as crosslinking agent and to the cured material obtained with the coating mixture.
Description of Related Art:
Polycarbodiimides are well known crosslinking agents for aqueous polymers containing carboxylic acid functions. A review of the development of the preparation and the application of polycarbodiimides is described in EP1644428 (U.S. Pat. No. 7,439,316). Of particular interest are the polycarbodiimides that possess additional functional groups that contribute to the crosslinking.
The preparation and application of multifunctional polycarbodiimides as crosslinking agents is described in EP0507407 (U.S. Pat. No. 5,258,481). These crosslinking agents contain carbodiimide groups and at least one other functional group.
Both groups contribute to the crosslinking. A relative large amount (about 50%) of solvent is used in the preparation method because the viscosity would otherwise become too high during the first stage of the preparation of the multifunctional polycarbodiimide crosslinking agent. As a result the crosslinking agent contains a relatively large amount of volatile organic compounds, which is undesired nowadays because of environmental reasons. In addition, the concentration of carbodiimide groups and the concentration of the other reactive groups are apparently relatively low by using this method, so that a relatively high amount of multifunctional polycarbodiimide crosslinking agent is required to obtain a good crosslinking degree of the polymer to be cross-linked.
It is described in the conclusions and examples of U.S. Pat. Nos. 6,566,437 and 4,118,536 how an alkoxysilane functional polycarbodiimide can be prepared from an isocyanate with an alkoxysilane function, but this polycarbodiimide does not contain hydrophilic groups and will as a result not be well dispersible in water or aqueous systems. Moreover, 36% of an organic solvent is used in the example of U.S. Pat. No. 6,566,437 so that the benefits of the present process are not disclosed. An example of U.S. Pat. No. 4,118,536 describes how an alkoxysilane functional carbodiimide can be prepared from an isocyanate with an alkoxysilane function, but the alkoxysilane functional carbodiimide concerned is made in a very low concentration in this example, so that the benefits of the present process are missing.
The object of the present invention is to provide a process in which the disadvantages mentioned above are eliminated.